1. Field of the Invention
The present invention relates to a drive circuit and a drive method of a light emitting display apparatus using light emitting display devices, especially using organic electro-luminescence (hereinafter briefly referred to as EL) devices.
2. Related Background Art
In recent years, the development of displays using self light emitting devices, such as an inorganic EL device, an organic EL device, and a light emitting diode, has actively performed as the next generation large-screen thin-shaped display apparatus followed by a liquid crystal display apparatus and a plasma display apparatus.
In particular, because the organic EL device can be formed by depositing layers on a glass substrate and a flexible substrate such as a film, the full-scale practical realization of the organic EL device has been expected from the recent improvement of the luminous efficiency and the reliability thereof.
The drive of the organic EL device is principally performed by an active matrix drive system, which has been proven in liquid crystal displays and is realized by thin film transistors (hereinafter referred to as TFTs) using poly-silicon, an amorphous semiconductor, or the like.
Among them, the amorphous semiconductor can be formed on a film substrate by a low temperature process, and thereby has a technological advantage in the use of large-size and thin-shaped displays.
The active-matrix (hereinafter referred to as AM) type organic EL display apparatus adjusts the brightness thereof and performs gradation display by controlling the voltages and the currents that are supplied to the organic EL devices according to voltage signals or current signals that are supplied on drive transistors (see FIG. 15). The U.S. Pat. No. 6,809,706 discloses an example of a pixel circuit of the AM type organic EL display apparatus including the drive transistors.
However, there is the following actual problem in controlling the brightness of the organic EL devices by means of the drive transistors.
That is, the electric characteristics of the drive transistors are sometimes dispersed in respective pixels owing to the variation of manufacturing processes. Moreover, the electric characteristics of the drive transistors sometimes change dependently on environments and electrification times. When the electric characteristics change, the changes of threshold values are especially remarkable.
Even if the same data signal is applied, the currents flowing through the drive transistors are consequently different from one another. The changes of the currents cause production of the differences of brightness of the light emitting devices at each pixel and each time, and therefore bring about display unevenness extending over the whole display screen.
On the other hand, it has been proposed to cancel the dispersion of the electric characteristics of the drive transistors by operating the voltages supplied to light emitting devices to have the same electric potential as the data signals by negative-feedback loops using differential amplifiers (see, for example, U.S. Pat. No. 6,809,706). In this case, the control of the brightness of the light emitting devices is performed based on voltages.
However, if the brightness of organic EL devices is controlled based on voltages, the gradation control of the devices is more complicated because the brightness-voltage characteristics of the devices do not have linearity.
Moreover, because the brightness-voltage characteristics change with time, the method of such a control is also needed to be changed according to the aged deterioration of the characteristics.
From the above reasons, it is desirable to control the brightness not using voltages but using currents.
Accordingly, it is an object of the present invention to provide a drive circuit realizing good image quality even if the threshold values of drive transistors show variation, or aged deterioration when stable currents are supplied to light emitting devices in a light emitting display apparatus. That is, according to a scope of the present invention, a drive circuit and a drive method of a light emitting display apparatus are both capable of realizing good image quality by correcting the threshold values of drive transistors when the light emitting operations of light emitting devices are performed at desired brightness gradation.